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arXiv2019

How IGN (France) computed the so-called 'centre of gravity' of physical Europe in 1989 and 2004

The method used by senior geodetic engineer Jean-Georges Affholder to determine what can be termed as the 'centre of gravity' of physical Europe in 1989 and 2004 relies on mathematical formulae which, in their only published version, happen to be flawed with typographical errors that do not reflect Mr. Affholder's actual mathematical exactness. This short epistemological paper summarizes the major steps of Affholder's method, provides a corrected version of the general formulae, and briefly recalls some particulars of the specific determination of the centre of gravity of physical Europe.

math.HO↗

arXiv2019

Fun in 2+1

To celebrate Roman Jackiw's 80th birthday, herewith some comments on gravity and gauge theory models in D=3, the chief focus of many of our joint efforts.

hep-th↗

arXiv2019

Diagnosing Disagreements: The Authentication of the Positron 1931-1934

This paper bridges a historiographical gap in accounts of the prediction and discovery of the positron by combining three ingredients. First, the prediction and discovery of the positron are situated in the broader context of a period of 'crystallisation' of a research tradition. Second, the prediction and discovery of the positron are discussed in the context of the 'authentication' of the particle. Third, the attitude of the relevant scientists to both prediction and discovery are conceptualised in terms of the idea of 'perspectives'. It will be argued that by examining the prediction and discovery of the positron in the context of authentication within a period of crystallisation, we can better understand disagreements regarding the positron between relevant scientists (Dirac, Bohr, and Pauli) in the period 1931-34.

physics.hist-ph↗

arXiv2019

Bruno Touschek in Germany after the War: 1945-46

Bruno Touschek was an Austrian born theoretical physicist, who proposed and built the first electron-positron collider in 1960 in the Frascati National Laboratories in Italy. In this note we reconstruct a crucial period of Bruno Touschek's life so far scarcely explored, which runs from Summer 1945 to the end of 1946. We shall describe his university studies in Göttingen, placing them in the context of the reconstruction of German science after 1945. The influence of Werner Heisenberg and other prominent German physicists will be highlighted. In parallel, we shall show how the decisions of the Allied powers towards restructuring science and technology in the UK after the war effort, determined Touschek's move to the University of Glasgow in 1947.

physics.hist-ph↗

arXiv2019

Michael Atiyah and Physics: the Later Years

I review aspects of Michael Atiyah's research in fundamental theoretical physics during the last ten years of his life, and contrast his deliberate interest in physics during this period with the inadvertent impact on physics of his early mathematical work.

physics.hist-ph↗

arXiv2019

On the Topic of Emergence from an Effective Field Theory Perspective

Effective Field Theories have been used successfully to provide a "bottom-up" description of phenomena whose intrinsic degrees of freedom behave at length scales far different from their effective degrees of freedom. An example is the emergent phenomenon of bound nuclei, whose constituents are neutrons and protons, which in turn are themselves composed of more fundamental particles called quarks and gluons. In going from a fundamental description that utilizes quarks and gluons to an effective field theory description of nuclei, the length scales traversed span at least two orders of magnitude. In this article we provide an Effective Field Theory viewpoint on the topic of emergence, arguing on the side of reductionism and weak emergence. We comment on Anderson's interpretation of constructionism and its connection to strong emergence.

physics.hist-ph↗

arXiv2019

Stern-Gerlach: conceptually clean or acceptably vague?

This paper develops a number of quantum mechanical characterisations of Stern-Gerlach. It discusses areas of vagueness in their formulation. Philosophers criticise quantum mechanics for unacceptable vagueness in connection with the measurement problem. The quantum formulation problems identified by this paper go beyond the locus of philosophical criticism. It concludes with an open question, are some areas of vagueness in quantum mechanics more acceptable philosophically than others and, if so, why?

physics.hist-ph↗

arXiv2019

Einstein and Eddington and the consequences of general relativity: Black holes and gravitational waves

For the celebrations of the 100 years of the observations undertaken by Eddington at the island of Principe and collaborators at Sobral during a total solar eclipse in May 29, 1919, which have confirmed Einstein's theory of general relativity through the deflection of the incoming light from distant stars due to the spacetime curvature caused by the Sun, we highlight the main aspects of the theory, its tests and applications, focusing on some of its outstanding consequences. These are black holes, the object par excellence of general relativity, and gravitational waves, the gravitational probe for the distant Universe. We also point out some open issues.

physics.hist-ph↗

arXiv2019

Everettian theory as pure wave mechanics plus a no-collapse probability postulate

Proposed derivations of the Born rule for Everettian theory are controversial. I argue that they are unnecessary but may provide justification for a simplified version of the Principal Principle. It's also unnecessary to replace Everett's idea that a subject splits in measurement contexts with the idea that subjects have linear histories which partition: Deutsch 1985, 2011; Saunders and Wallace 2008; Saunders 2010; Wallace 2012, Chapter 7; Wilson 2013, forthcoming. Linear histories were introduced to provide a concept of pre-measurement uncertainty and I explain why pre-measurement uncertainty for splitting subjects is after all coherent, though not necessary because Everett's original fission interpretation of branching can arguably be rendered coherent without it, via reference to Vaidman 1998; Tappenden 2011; Sebens and Carroll 2018; McQueen and Vaidman 2019. A deterministic and probabilistic quantum mechanics can be made intelligible by replacing the standard collapse postulate with a no-collapse postulate which identifies objective probability with relative branch weight, supplemented by the simplified Principal Principle and some revisionary metaphysics.

quant-ph↗

arXiv2019

Homogeneity and the causal boundary

A Universe with finite age also has a finite causal scale $χ_§$, so the metric can not be homogeneous for $χ>χ_§$, as it is usually assumed. To account for this, we propose a new causal boundary condition, that can be fulfil by fixing the cosmological constant $Λ$ (a free parameter for gravity). The resulting Universe is inhomogeneous, with possible variation of cosmological parameters on scales $χ\simeq χ_§$. The size of $χ_§$ depends on the details of inflation, but regardless of its size, the boundary condition forces $Λ/8πG $ to cancel the contribution of a constant vacuum energy $ρ_{vac}$ to the measured $ρ_Λ\equiv Λ/8πG + ρ_{vac}$. To reproduce the observed $ρ_Λ \simeq 2 ρ_m$ today with $χ_§\rightarrow \infty$ we then need a universe filled with evolving dark energy (DE) with pressure $p_{DE}> - ρ_{DE}$ and a fine tuned value of $ρ_{DE} \simeq 2 ρ_m$ today. This seems very odd, but there is another solution to this puzzle. We can have a finite value of $χ_§\simeq 3 c/H_0$ without the need of DE. This scale corresponds to half the sky at $z \sim 1$ and 60deg at $z \sim 1000$, which is consistent with the anomalous lack of correlations observed in the CMB.

gr-qc↗

arXiv2021

What? Who? Why? Stellify

In his 1981 article, Roberts highlights the term 'stellify' defined as "to transform (a person or thing) into a star or constellation, to place among the stars." Using the case of the Tabwa people of central Africa, not the Democratic Republic of Congo, Roberts presents among other things the sky as a mnemonic for remembering migrations and remembering culture heroes. We do not know the details of the processes of stellification, however we do know what has been stellified in many cultures by examining their names for stars and asterisms and their skylore. Of the many ideas presented in his latest book, Aveni teases out the ideas of the sky stories having connections to celestial motions, as well as being a mnemonic for remembering seasonal activities and a mnemonic for remembering locally embedded moral, ethical, and sociocultural codes, thus overlapping with Roberts' supposition of the sky serving as a mnemonic. I draw on case studies to flesh out three themes 1. celestial motions, 2. moral, ethical, and sociocultural codes, and 3. seasonal activities within African sky stories. As previously stated, though the human process of assigning names and stories to the night sky as well as stellifying aspects of their lives is not fully understood, these three themes hold promise for being foundational if not part of every culture's practice of stellification.

physics.hist-ph↗

arXiv2021

A Commentary on {\it The Knowledge Machine} by Michael Strevens

We offer a few comments derived from a careful reading of Michael Strevens' book {\it The Knowledge Machine} (TKM), with an emphasis on extensions for future work. We believe this book goes well beyond traditional accounts of scientific change, and offers thus many insights into new research.

physics.hist-ph↗

arXiv2021

Lack of Value Definiteness in Quantum Gravity

In this paper we consider the observables describing fundamental spatiotemporal properties and relations in the context of Quantum Gravity (QG). As we will show, in both Loop Quantum Gravity and in String Theory, these observables are non-commuting ones. By analogy with some recent arguments put forward in the context of non-relativistic quantum mechanics (QM), we shall propose to interpret these physical quantities as ontologically indeterminate -- i.e., indeterminate in a non-epistemic, mind-independent way. This result has two important consequences for current debates in philosophy of physics. First, it shows that ontological indeterminacy may extend well beyond the non-relativistic case, thereby also suggesting a conceptual continuity between QM and QG. Second, when applied to QG, the notion of indeterminacy may provide a unified philosophical framework for understanding very distinct approaches that are usually considered incompatible.

physics.hist-ph↗

arXiv2021

Collapse of the many-worlds interpretation: Why Everett's theory is typically wrong

We analyze the objective meaning of probabilities in the context of the many-worlds interpretation of Everett. For this purpose we study in details the weak law of large numbers and the role of typicality and universally negligible probabilities (through the works of Cournot and Borel). We demonstrate that Everett's theory doesn't provide any clue for fixing a probability rule and therefore contradicts irrevocably empirical facts and Born's law.

quant-ph↗

arXiv2021

The Fine-Tuning of the Universe for Life

When a physicist says that a theory is fine-tuned, they mean that it must make a suspiciously precise assumption in order to explain a certain observation. This is evidence that the theory is deficient or incomplete. One particular case of fine-tuning is particularly striking. The data in question are not the precise measurements of cosmology or particle physics, but a more general feature of our universe: it supports the existence of life. This chapter reviews this Fine-Tuning of the Universe for Life.

physics.hist-ph↗

arXiv2021

Reluctant Pioneer of Nuclear Astrophysics: Eddington and the Problem of Stellar Energy

During the years from 1917 to 1921, A.S. Eddington was intensely occupied with Einstein's general theory of relativity and the epic eclipse expedition which confirmed one of the theory's predictions. During the same period, he investigated the old problem of why the stars shine, which led him to suggest two different subatomic mechanisms as the source of stellar energy. One of them was the annihilation of matter and the other the building-up of helium from hydrogen. This paper is concerned with Eddington's work in this area, a line of work to which he returned on and off during the 1920s but then abandoned. His decision to stop working on the stellar energy problem coincided with the first attempts to understand the problem in terms of nuclear physics and quantum mechanics. Why did Eddington not follow up his earlier work and why did he ignore the contributions of the nuclear physicists which in the late 1930s resulted in the first successful theories of stellar energy production?

physics.hist-ph↗

arXiv2021

Drawing Scales Apart: The Origins of Wilson's Conception of Effective Field Theories

This article traces the origins of Kenneth Wilson's conception of effective field theories (EFTs) in the 1960s. I argue that what really made the difference in Wilson's path to his first prototype of EFT are his long-standing pragmatic aspirations and methodological commitments. Wilson's primary interest was to work on mathematically interesting physical problems and he thought that progress could be made by treating them as if they could be analyzed in principle by a sufficiently powerful computer. The first point explains why he had no qualms about twisting the structure of field theories; the second why he divided the state-space of a toy model field theory into continuous slices by following a standard divide-and-conquer algorithmic strategy instead of working directly with a fully discretized and finite theory. I also show how Wilson's prototype bears the mark of these aspirations and commitments and clear up a few striking ironies along the way.

physics.hist-ph↗

arXiv2021

Identity and difference: how topology helps to understand quantum indiscernability

This contribution, to be published in Imagine Math 8 to celebrate Michele Emmer's 75th birthday, can be seen as the second part of my previous considerations on the relationships between topology and physics (Mouchet, 2018). Nevertheless, the present work can be read independently. The following mainly focusses on the connection between topology and quantum statistics. I will try to explain to the non specialist how Feynman's interpretation of quantum processes through interference of classical paths (path integrals formulation), makes the dichotomy between bosons and fermions quite natural in three spatial dimensions. In (effective) two dimensions, the recent experimental evidence of intermediate statistics (anyons) (Bartolomei et al. 2020) comfort that topology (of the braids) provides a fertile soil for our understanding of quantum particles.

physics.hist-ph↗